In recent years, flat panel displays, which are typified by a liquid crystal display device or an electroluminescence display device (hereinafter, referred to as an EL display device) including an electroluminescence element, are under mass production as the mainstream of the image display device.
In an active matrix liquid crystal display device or in an active matrix EL display device, each pixel in a pixel portion is provided with a transistor. These transistors include a semiconductor layer of silicon (Si) as an active layer.
In contrast, there has been proposed is an image display device in which a transistor includes oxide in its active layer (e.g., see Patent Document 1).